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-   -   Stokes Drag (http://www.cfd-online.com/Forums/fluent/32641-stokes-drag.html)

Ajay Rao December 3, 2003 15:18

Stokes Drag
 
Hello everyone ,

I am trying to calculate the stokes drag on a sphere held stationary in a flow at reynolds number = 0.1 particle size is 2mm dia and the outer domain I have is 12 dia ( R2/R1 = 24 ). Stokes drag claculated using the formula Drag=6*pi*Meu*radius*U0

U0 = 5.024e-5 based on Re Stokes Drag= 9.48e-10 N

FLUENT gives me the Drag ( I use Report -> x dir force ) as

a) 6.29 e-9 N when I do a steady case, initializing the velocity as 0 . Applying BC ( Setting X dir velcoity ) on the outer domain with U0=5.024e-5.

b) 1.81e-10 N when I do a steady case but initialize the velocity as 5.024e-5 and use the same BC as above.

c) 1.06e-9 when i do an Unsteady case and I run the simulation for 50 seconds.

But in any case its not close enough to 9.48e-10 the stokes drag . Any ideas as to why i am off ??and what to do to get the exact stokes drag ?

Thanks -A

ccc December 4, 2003 12:04

Re: Stokes Drag
 
hi Ajay,

have you raised the convergence criterion to see if results change?

good luck

Ajay Rao December 4, 2003 17:41

Re: Stokes Drag
 
yes i tried this , now my error is around 8 % ie FLUENT drag is 8 % higher than the actual stokes drag. I dont know if that is exceptable. I thought the stokes drag could be predicted quiet accurately. Thanks for the advice, Ajay

ccc December 5, 2003 04:18

Re: Stokes Drag
 
hi Ajay,

have you checked the indepence of grid and the size of domain?

good luck.



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